A 1964 article in Nature with the euphonious title, “Nature of argillaceous odour,” gave the world the not-as euphonious-sounding word, “petrichor.” In it, two researchers attempted to scientifically describe what it is we smell when we smell the world after a rain shower and to give it a name.

The two authors coined the word, “petrichor,” which I have been mispronouncing in my head since I first encountered it last year, when an article on the Huffington Post started making its social media rounds. It has a long “I,” so say it like this: “petra,” then “eye-core,” which is not how I hear it in my head, with a short “i.”

It is one of the most familiar of smells, pungent, a little clayey, the essence of spring and summer, yet until 1964 and even more recent experiments, no one knew exactly what that smell is. In 1964 and ’65, the two researchers, one Australian and one British, wrote a pair of articles for the science journal Nature in which they outlined a theory about that post-rain odor. A hint is in their title, “argillaceous,” which simply means “things related to or having to do with clay.” (This will not stop me from saying the word the next time I eat something that I really enjoy. It sounds like a compliment. “You’ve outdone yourself. This was truly argill-” and then my girlfriend will kick my shin to prevent me from insulting the cook.) Before 1964, the word “petrichor” did not exist, and every single writer who attempted to evoke that post-rain scent discovered that they were stuck with the phrase “post-rain scent.”

Their theory, that it is a bunch of compounds that are never exposed to our olfactory equipment during any other weather condition, may strike one as being a little obvious. It isn’t, though. In dry weather, the authors wrote, certain plants, certain trees, excrete oils that are absorbed by the more clayey soils around them. These oils have a job: they help slow seed germination during dry seasons, when new plants might face a harsh start to life. During a rain shower, the soil yields up some of these oils, now no longer needed to protect seeds, along with another substance, geosmin, which is a waste product of certain bacteria in soil. This specific combination of molecules is what we smell, they wrote.

“Petr-” means rock or stone (petrology). “Ichor” is the blood of the gods in Greek mythology. Thus, petrichor is quite a poetic term; if “the blood of the gods released from stone” is what Isabel Bear and Roderick Thomas were reaching for, well, that smell indeed is rare and fine to most human noses.

The results of an MIT study were released last winter, further confirming some thoughts about petrichor along with some surprises. A high-speed camera was used to photograph simulated rain against different soils. The Huffington Post article has “Crazy Slo-Mo Video Explains Why Rain Has That Distinctive Smell” as its headline, which is a bit more eye-catching than the MIT news release about the study: “Rainfall can release aerosols, study finds.” The study’s authors set up 600 experiments on 28 different surfaces and discovered that raindrops “fizz” on impact. (My word, not theirs.) This was something that no one had even theorized.

The images they produced revealed a mechanism that had not previously been detected: As a raindrop hits a surface, it starts to flatten; simultaneously, tiny bubbles rise up from the surface, and through the droplet, before bursting out into the air.

This had not been seen until this year, but almost every human nose has registered it since our ancestors first encountered rain. The researchers further verified something that every human being who has stood outdoors in a light rain has noticed but never verified with “crazy slo-mo” tools of any kind: that a light rain striking clayey soils releases a lot more bubbles per raindrop. A heavy rain drowns each raindrop; a light rain, like the kind I saw in the woods out behind my house when I was a child, a light rain striking the leaves and branches of trees, further slowing their impact, that rain produces the strongest petrichor of all and is the one that renders me into an seven-year-old noticing the world for the first time.

The lightest of rain after the driest of spells leads to the most argillaceous petrichor, which is the kind that humans smell as relief, the thought that things will start growing again.

For those (like me) with sleep disorders, here is a video of five hours of rain sounds. You can almost smell the petrichor.
 

  

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This first appeared in January 2015 then in June 2015, in November 2015, and last March.

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